Our recent work has shown that the mitochondrial coupling factor B (FB), a 15,000 dalton -SH containing protein, is required for the Pi-ATP exchange activity of the energy transducing ATPase complex, but not for F1 binding to the membrane or the oligomycn sensitivity of the membrane-bound F1. Further characterization of the chemical properties of FB, its location in the membrane and its relationship to the other proteins of the ATPase complex will be studied in order to get better insight into its mechanism of action. Purification of the lipid-containing ATPase complex is proposed with the aim of removing several minor contaminants. Those membrane sector proteins of the ATPase complex which have not been purified so far, will be separated and purified. The role of FB will be explored by incorporation of the membrane proteins indifferent combinations into liposomes and assay of their activities with and without FB in several reactions.